Field of the Invention
This invention relates to a driving method of a liquid crystal display device and, more particularly, a driving method suitable for an active-matrix type liquid crystal display device.
An active-matrix type liquid crystal display device (called hereinafter the "AM LCD") generally includes an array substrate, a counter substrate provided opposite to the array substrate with a predetermined gap and a liquid crystal layer disposed in the gap. The array substrate is provided with a glass plate, a plurality of signal lines made of a low electric resistance material, such as aluminum, a plurality of scanning lines made of tungsten molybdenum (MoW) crossing the signal lines in a matrix form, and thin film transistors (called hereinafter the "TFTs") disposed in the vicinities of the cross points thereof as switching elements. The signal and scanning lines and the TFTs are formed on the glass plate. Pixel electrodes connected to the TFTs are disposed at pixels surrounded by the signal and scanning lines. The pixel electrodes are made of indium tin oxide (ITO). An alignment layer is formed to cover the pixel electrodes on the glass plate. The counter substrate, on the other hand, includes a counter electrode made of ITO on the inner surface of which an alignment layer is formed. Japanese Patent No. 2523587 discloses an AM LCD which includes a dummy line in addition to the scanning lines and capacitors defined by pixel electrodes at the top and bottom rows and the dummy lines. The dummy line is formed in the same layer as in the scanning lines and is supplied with a predetermined voltage.
Where a driving method is adapted to compensate for punching-through voltages in the AM LCD, significant shifts occur with respect to the optimum common voltages for one region of pixel electrodes to define the capacitors with the dummy line and for another region to define capacitors with the scanning lines. As a result, the brightness of images in the former region is different from that in the latter region so that the overall image is not satisfactory in display quality.